I think it boils down to a religious stance on 'thou shalt not kill'....'it is God's domain to deal with sinners'....and the ever fashionable 'murder in any form is reprehensible'......
That said, you won't see me publicly advocating harm to these murderers for obvious reasons, but you won't see me lamenting their demise by others, either....
Not very many prolifers are full-blown pacifists, which would at least make their position consistent.
Probably most believe war is sometimes justified, that killing in defense of self or other is justifiable homicide. Probably most, in this country, believe in capital punishment.